In the radio frequency (RF) communication area, a self quieting problem exists when a data processor is utilized. The problem is that the data processor generates a system clock that is a pulse train having harmonics of its fundamental frequency. These harmonics radiate from the data processors section of the circuitry into sensitive receiver sections. This causes the self quieting problem.
The self quieting problem occurs when frequency modulation (FM) receiving equipment locks onto a channel which is on the same frequency as a harmonic of the fundamental clock frequency. The clock harmonic over powers the received signal in this case, giving the impression of a busy channel with a dead carrier being received (quiet). This is especially detrimental to the carrier detecting channel scan systems which will stop on clock harmonics instead of valid busy channels.
There are currently three solutions to this self quieting problem: grounding and shielding of unwanted radiation, clock frequency shifting, and frequency spreading. Grounding and shielding accomplish an attenuation by blocking and/or shorting the interference. While grounding and shielding function adequately, a high degree of attenuation generally requires relatively expensive parts and a great deal of manufacturing time. This adds significantly to the cost of the final product.
The second alternative of clock frequency shifting moves the problematic harmonics out of the receiver input range selectively such that each desired receive frequency requires custom configuration of a multi-frequency clock generator. This requires preprogramming of the system which requires that channel operations of the system be known in advance. Clock frequency shifting effectively moves the problem to another frequency and does not accomplish interference power reduction.
The third alternative of frequency spreading involves clocking the data processor by a frequency modulated clock generator where the modulation input is a waveform that causes the output spectrum to be spread over a relatively wide range of frequencies resembling a pink noise spectrum. The power spreading serves to reduce the peak power of the harmonics due to the power conserving nature of frequency modulation and, when peak power is reduced below the noise floor level of the receiving equipment, the problem is eliminated. Currently, implementations of this alternative require the additional waveform generating circuitry.